Do you NEED 8GB, No
Will 8GB help with some games and apps ETC., Yes.
I have 4GB and max used was about 3.25GB Skyrim with mods, but I believe if you allow games more RAM they will take advantage of it.
If you do get more RAM go for 1866MHz+, 1866 is becoming the new normal as with Haswell it will native support 1866, so 1600 will less available later. Also more kit's are identical or about $5 more to go from same size to 1600->1866

I have 4GB and max used was about 3.25GB Skyrim with mods, but I believe if you allow games more RAM they will take advantage of it.

Click to expand...

It uses more than that. Because you only have 4GB of RAM the OS scales back the maximum amount of applications cached to optimise it for a malnourished system. Likewise, because of the RAM shortfall games like Skyrim don't have access to enough System RAM for when the VRAM depletes so it makes do with 3.25GB and borrows from the HDD where necessary. Doesn't mean 4GB is all that's needed, just means your computer is good at auto optimising itself for a deficiency.

I done a print screen of my resources after a restart (so you guys can't use the memory leak excuse on me)

This screen shot is after 10minutes of playing Crysis 2, maximum detail @ 1440x900.

As you can see it's using almost 6GB of physical memory (5.49GB). Had my resolution been bigger I'm sure more RAM would be needed.